Means of controlling wing tip stall in airplanes



I. L. ASHKENAS MEANS OF CONTROLLING WING TIP STALL IN AIRPLANES 4 SheetsSheet 1 INVENTOR. be w/va [.451/4 54/45 fizzarrzg/ April 17, 1951 Filed Jan. 26, 1948 9 array 1. L. ASHKENAS 2,549,045

MEANS OF CONTROLLING WING Ti? STALL IN AIRPLANES V 4 Sheets-Sheet 2 INVENTOR. Lew/v0 L Asdzams fit April 17, 1951 FiIQed Jan. 26, 1948 MEANS OF CONTROLLING WING TIP STALL IN AIRPLANES Filed Jan. 26, 1948 4 Sheets-Sheet 4 NOSE NOSE DOWN NosE CM c$ i A? NOSE Down PLAIN mm. A a

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-- SHAFT Housme wmG mo PROP. SHAFT NOSE l-lousma AND sun C INVENTOR- Dowu lemva L. ASA ems fi/forney Patented Apr. 17, 1951 MEANS OF CONTROLLING WING TIP STALL IN AIRPLANES Irving L. Ashkenas, Beverly Hills, Calif., assignor to Northrup Aircraft, Inc., Hawthorne, Califi, a corporation of California Application January 26, 1948, Serial No. 4,414

position of the center of gravity with respect to the center of lift, or the neutral point. In a basic wing type aircraft, the neutral point may be shifted aft by adding a tail or, preferably, by

sweeping the wing. The center of gravity can be shifted forward by proper weight distribution, so

from the standpoint of static longitudinal stability, no particular configuration has any especial advantage, except as it affects the possibility of proper balance.

In an all-wing airplane, the elimination of the tail makes the problem of balance somewhat critical, even though the wing panels are sweptback, although not excessively so. Unfortunately, however, for any given airplane, the neutral point does not ordinarily remain fixed with variations of power, flap setting, or even lift coefficient, so that the aft center of gravity limit for stability is often prescribed by some single flight condition. Extensive experience by the present inventor and his associates with tailless aircraft has shown that the critical condition for longitudinal stability has always occurred for poweroff flight at angles of attack approaching a stall, i. e., during landing maneuvers.

The experience mentioned above has made the pitching instability of a swept wing a familiar phenomenon. The complete mechanisms involved, however, are still somewhat obscure. There are apparently two opposing effects which are of prime importance. They are, the tendency for sweep-back to increase the relative tip loading and also (by creating a span-wise pressure gradient) to promote boundary layer flow toward the wing panel tips.

On a clean swept-back wing the latter effect apparently nullifies the former so that there occurs in the tip portion of the wing panel a gradual decrease in effective sectionlift-curve slope with a resulting progressive decrease in stability. Under these conditions, however, the tip never completely stalls, as will be more fully pointed out later. i

On a swept-back wing panel, however, which has had any basic modifications made thereon 7 Claims. (01. 244-42) 2 that affects the span-wise flow, there will be a noticeable eifect on the pitching behavior at high lift coefficients, as the normally expected tip stall will be produced, as evidenced by strongly unstable moments in the vicinity of the maximum lift coefficient.

One of the basic modifications, for example, of an all-wing craft, in the above respect, par ticularly when pusher propellers are used, is the propeller shaft housings. These housings act to inhibit span-wise flow somewhat and to tend to straighten out the moment curve below the stall, but to increase the unstable tendencies at the stall. Consequently, in order to obtain stability at the stall, other means must be resorted to, and it is an object of the present invention to provide a means and method of increasing the stalling angle of the wing tip sections.

I have found that a tip slot can be utilized to increase the stalling angle of the tip sections.

However, experience has also shown thatit is not always possible for a pilot to detect the approach to a stall. In consequence, it is another object of the present invention to provide a means and method of automatically opening the normally closed tip slots as a stall is approached, preferably at predetermined values of lift coefficient of the wing tips.

The possibility of controlling the stalled. portions of the wing, as outlined above, means that trailing edge flap controls can be laid out to maintain their effectiveness at high angles of attack, and this effect can be efficiently utilized, such a flap layout, however, forming no part of the present invention.

My invention can be more fully understood by reference to the drawings, showing a preferred form of the invention as applied to an all-wing army bomber.

In the drawings:

Figure 1 is a perspective view of part of an allwing army bomber utilizing the automatic slot control of the present invention.

Figure 2 is a fragmentary cross sectional view taken as indicated by the line 2-2 in Figure 1.

Figure 3 is a diagrammatic view from above showing closure linkages.

Figure 4. is a diagrammatic sectional view of a pressure switch.

Figure 5 is a wiring diagram showingthe circuit operating the slot closures in accordance with the condition of the pressure switches.

, Figure 6 is a coordinate diagram of the pitching characteristics at high'lift of a clean sweptback wing.

Figure 7 is a similar diagram relating to a swept-back wing with end plates.

Figure 8 is a similar diagram incorporating the moment-lift curves of Figures 6 and 7 with a curve for a wing having propeller housings and a tip slot.

Referring to the drawings: 7

In Figure l, onewing panelWof an all-wing army bomber known as the B-35 of 172 ft. wing spread is shown, the wing panels being sharply swept back. Such an airplane may be driven by contra-rotating propellers I from engines housed within the wing panels through prqpeller shaft housings 2, cooling of these engines-beingaccome plished through a leading edge air intake 3. Nor.- mal control of the airplane is. by elevons .4 ae= tuated either simultaneously or independently,

and split rudders 5 for producing unilateral drag.

at the wing tips. Large area landing flaps 5 arepositioned between the propeller housings 2 and the crew nacelle 1.

To, prevent wing tip stall, a wing tip slot H3 'is positioned near the leading edge of each wing panel adjacent the wing panel tip, this slot being shown in more detail in Figures 2 and 3.

Referring to these latterfigures, the slot starts from under wing surface II and curves rearwar lly to open on theupper wing surface 62, the

, S .0t beingldefined within the wing panel by slot wallsi3. slot is is closed at one end by a flush lowersurface door or closure M and at the other end b an l persurface door or closure I5, Clo- ;suresil and i5 arevrearvvardly pivoted on axles l6 and r s cti y and i e nwar l nt 4 is a unique function of the wing lift coemcient, so that with proper adjustment of the bellows area or the fulcrum point 42 the pressure switch can be used to indicate a predetermined lift coefficient by connecting bellows 31 to one end of a lever 41 outside of compartment with bellow-s 33 connected to lever 4| on the opposite side of lever axle 42. A microswitch 43 is then positioned to be actuated by one end of lever 4i, At the predetermined pressure ratio the direction of the force acting on lever M will re verse, tending to compress the lever spring 44 (positioned to normally maintain the switch open) and thereby to close the switch 43.

Of the two pressure switches on each wing panel, one, tube referred to as trip switch 43T,

is set to close at a lift coefficient of 0.6. The other switch, hereafter to be known as reset switch 43R, is adjusted to close at a lift coeffici nt o 0.7. r

Having described the slot opening and closing mechanism and the lift coefficient switches, ref erence will next be made to Figure 5 showing one type of fail safe electrical operating circuit for slot door actuation in accordance with changes in lift 'coefiicient, although the actual circuit Shown is no part of the present invention. Other means well known in the art of obtaining the desiredsjlotresponse can be utilized.

The power supply 155 forthis preferred circuit .24 v. D. C. with the negative pole grounded.

The positive power lead5i goes directly to a pilots switch 52 havin three positions; slot doors open; slot doors. closed; and automatic actuaof ha lmar s B a c nnect d-t u per door .15. by upper actuating rod 2|, and to the lower door 14 by lower actuating rod 22 the latter being. spring loaded as by spring 23.

All of thebell cranks 20 are operated by a connetting rod 25 as shown in Figure 3, one bell crank i lilo being rotated by lever link 26 driven by the piston rod 2.? of a hY-draulic motor 28 under control of an electrically operated hydraulic solenoid yalveail. A strong emergency-slot opening'spring 3 1 isattached to piston rod 21. Hydraulic valve 13.0 is constructed to hold the doors i i and I5 Closed against spring 3| by continuous application of hydraulic pressure in motor 28. Thus, lossof hydraulic pressure to the motor 28 would enable spring 5| to open doors I4 and I5.

. Eaehof thfl Win panels is provided with a slot, andslot opening and closing means as described, and,;in addition,-is provided with two pressure switches adjacent each of the slots as shown in Figure 4, each of the pressure switches-comprises aoo npartment v35 connected to an air scoop 36 extending below the boundary layer on-the lower wing surface I I. This provides total headpressure referred to hereafter as P3. Inside compartment, 3,5 is an u per surface pressure bellows 3's and a lower surface pressure bellows 38 respectively connected to upper wingsurface l2 by pipe 39, and lower wing surface II by a second pipe '40. The developed pressures are herein designated as P1 and P2 respectively, and are preferably picked up by pipes 39 and 40 at the 16% chord line. The pressure ratio of the pressures acting on bellows 31 and 38 tion. For the slot door open conditions, switch :2 .,merely opens the power lead 5|. For slot doors closedcondition, power-is transmitted by solenoid valve lead 53. directly to one terminal 54% of a pair'of -powerrelays 55 and 56and thence directly to solenoid valve windings 5'! through winding connection 58. Solenoid windings?! op- .erate both of'the'hydraulic motor valves 30 described above,-both windings being in parallel.

For automatic operation, switch 52 connects directly to a landing'gear switch 60 in series with V power lead 5|, this landing gear switch being closed when the landing gear is retracted, but

open when the landing gear is extended. Thus, under the fail safe conditions the slots always are open when the landing gear is extendedeven if the pilot should forget to open them manually.

From the landing gear switch 60 the power lea'cl extends to each pair of wing positioned pressure switches 43'I and 43R through leads GI and 62.

.' Describing the installation on one side only, lead 6| terminates in one contact of switches 43T and 43R on the same side which are thus bothtied together by link L. The other contact 63 of reset switch 43B is connected to winding 55 of one power relay and then to ground, and also to winding-54 of a latch-in relay 65, through latchin relay contact 66, and then to ground. The

. other contact 61 of the latch-in relay 55 is connected to the remaining contact 68 of trip switch 43T. On the opposite wing panel, switches 43R :and .43T are similarly connected, but in this case,

51 are deenerg'ized. This completes the circuit 1 description.

Inautomatic operation, when the pilots switch '52 is :placed in automatic position, and with the landing gear retracted, the right hand and left hand solenoid valve windings 51 will energize to cause the slot doors I and I5 (Figure 2) to close the slots against the spring 3| by action of hydraulic motor 28. This would be the normal flying condition. Opening of the power circuit 5| by any means, such as:

. By actuation of pilot switch 52;

. By actuation of landing gear switch 60;

. By failure of power supply;

. By closure of both reset and trip switches on either side;

marrow not energized, neither of the power relay windings 55 or 56 are energized, so solenoid windings 51 remain energized and the slot doors remain closed. When the same wing panel reaches a lift coefficient of 0.7 the reset switch MR closes, energizing the latch-in relay winding 64 which causes one of the power relaywindings 55 or 56 to be energized, thus breaking a bridge 13 or 14 in the power circuit 5l-l5--58 to the solenoid valve windings, whereupon the doors move. to open both slots. As long as the lift coefficient remains at 0.7 or above on one wing panel, the doors will remain open. When the lift coefficient is reduced below 0.7, the reset switch opens, but the doors still remain open since;- the latch-in relay remains energized through the trip switch 43T and the power circuit remains open. When the lift coefficient is reduced below 0.6, the trip switch 43T opens, deenergizing the latch-in relay 65 and the power relay, thus permitting the slot doors to return to the closed position.

From the above description, it will be seen that the slots are opened at a lift coefiicient of 0.7, but do not close until a lift coefficient of 0.6 is reached. Also it is to be noted that both wing tip slots open when the lift coefi'icient of 0.7 is reached even on one wing tip, but that the doors cannot close until the lift coeificient is below 0.6 on both wings. This method of handling the wing tip slots, together with the complete fail safe arrangement of slot controls, make for maximum safety near the stalling point with a minimum of pilot supervision.

The efi'ect of the various factors mentioned above on the static longitudinal stability of a tailless aircraft having swept-back wing panels is shown in the diagrams of Figures 6, 7 and 8.

In Figure 6, the pitching characteristics at high lift is shown with relation to the moment coefficient Cm and lift coeflicient Cl for a clean swept-back wing panel Wc. Here it will be noticed that the tip never completely stalls, as is evidenced by the stable pitching moments shown that occur at the maximum lift coeflicient. 0n the other hand, as illustrated in Figure 7, the use of a wing panel Wp having end plates to reduce to a large extent the effects of spanwise flow, straightens the pitching moment curve, but produces the normally expected tip stall, as evidenced by the strongly unstable moments in the vicinity of the maximum lift coeificient. Thus,

6 it is clear that any modification to'the basic wing which affects the spanwise flow will have a noticeable effect on the pitching behavior. at high lift coefficients.

The effect of propeller shaft housing, for example, is clearly shown in Figure 8 where the moment-lift curve A is shown for a clean sweptback wing. Curve B shows the eifect of the shaft housings, and curve C gives the overall effect of a wing tip slot when used a a means for controlling tip stall. The stability introduced by the wing tip slots is clearly shown.

While the present invention has been de-- scribed as applied to airplanes of the tailless type, it will be clear to those skilled in the art that the means and method of controlling stall are applicable to any airplane where slots can advantageously be used to increase the stalling angle. Furthermore, it is also clear that the device herein made illustrative of the invention is of advantage to control tip stall regardless of how the tendency for tip stall is accomplished.

What is claimed is;

1. In an airplane having laterally extending wing panels, a lift increasing slot adjacent the leading edge of the tip portion of each of said wing panels, upper and lower closures for said slots, closure actuating means, an operating member, closure controlling means on each wing tip for moving said operating member in accordance with the ratio of the difference between dynamic pressure generated by the airstream past the wing tip and the pressure of the boundary layer above the wing tip, to the difference between said dynamic pressure and the pressure of the boundary layer below the wing tip, said member being connected to actuate said closure actuating means at predetermined values of said ratio.

2. Apparatus in accordance with claim 1 wherein said closure controlling means include a bellows connected to the boundary layer above the wing panel tip, a second bellows connected to the boundary layer below the wing panel tip, and an enclosure for both bellows connected to an air scoop positioned outside of either boundary layer, and wherein said member is a lever connected by a link to each of said bellows whereby said lever is moved by one of said bellows or the other in accordance with the respective pressure differentials to which said respective bellow are subjected.

3. Apparatus in accordance with claim 1 wherein each of said operating members is connected to actuate both of said closure actuating means so that at a predetermined value of said ratio on either wing tip both of said closure actuating means will be actuated.

4. In an airplanehaving laterally extending wing panels, a lift increasing slot adjacent the leadin edge of the tip portion of each of said wing panels, upper and lower closures for said slot, a, closure actuating motor for simultaneously opening and closing said slot closures, a power source, motor actuating means between said source and said motor, means on each wing tip for measuring the ratio of the difference between dynamic pressure generated by the airstream past the wing tip and the pressure of the boundary layer above the wing tip, to the difference between said dynamic pressure and the pressure of the boundary layer below the wing tip, said latter means bein connected to operate said motor actuating means to open said slot when said ratio 7 rises above a predetermined value and to close said slot when said ratio falls below a predetermined value.

5. Apparatus in accordance with claim ewhere- V in both of said motor actuating means are connected to be operated simultaneously in response to a variation in either direction from said predetermined value of the pressure ratio measured by either of said pressure, ratio measuring said motor and normally in a position to cause, said motor to hold said closures closed when said motor is energized through said value, elastic meansurgin'g said closures to open position in the absence of motor energization, and closure controlling means in the tip of each wing panel includin a bellows connected to the boundary layer above the wing panel, a second bellows connected to the boundary layer below the Wing panel were tip, an enclosure for both bellows connected to an airscoop positioned outside of either boundary layer. a lever connected by a link to each of :said bellows whereby said lever is moved by one of said bellows or the'other in accordance with the respective pressure differentials to which said respective bellows are subjected, and electrical contacts actuated by said lever, said contacts 'berug-connected to actuate said solenoid value to open said closures at a predetermined lever position.

'7. Apparatus in accordance with claim 6 wherein the contacts actuated by'both levers are connected to operate both solenoid valves at a predetermined position of either lever.

' IRVING LWASHKENAS.

Number Name 1,351,538 Reynolds' Aug. 31, 1 920 2373;089 Allen et a1. Apr. 10, 1945 2,386,288 Blaylook a Oct. 9, 1945 

